Archive for the 'Prototyping' Category
As part of my mintlite project, I need to deal with an SO8 package battery charger. So I whipped up a breadboard adapter aka “breakout board” that converts the tiny chip into a 600 mil DIP package. Another battery charger I have is even smaller, packaged as an SOT23-5 part. The breakout converts that tiny grain of rice into a 300 mil DIP package.
Here’s a snapshot:
Cadsoft Eagle BRD files:
Preassembled, these adapters can cost upwards of $6 ea… as “kits”, they’re $1-2 … so here ya go, it’s like printing money!
As sort of a follow-up to my Inket Photolithography article, I have some more pictures of the process, from start to finish.
Using paintshop pro, I take individual PCB layouts and combine them into a composite panel, which is the exact size of the PCB I’m going to expose. Often I’ll repeat the same design a few times, in case there’s some glitch in one of them, or the board is damaged during the depanelizing process. I like to start with a black background for the panel, to mask out any unused sections of copper… this helps cut etching times and helps save the etchant life.
The inkjet transparency comes with a sheet of white paper attached, to protect the transparency and give the printer something extra to grab onto with the feed rolls. It takes about 30 min for the ink to fully dry.
This is my darkroom which also doubles as my house’s furance room. The exposure system is two cheap-o under cabinet lights, with “daylight” bulbs loaded in them. The timing is completely manual and is just an extension cord I unplug when time’s up.
After exposure, the PCB takes a bath in a sodium hydroxide solution to develop the resist layer. Within a few minutes resist that was softened from exposure to light is dissolved away. I rinse the board in hot water (per manuf. spec) to check for complete removal of the resist – sometimes a thin film remains and another dip in developer is required. After all is well, a rinse in cold water sets the resist and halts the developing process.
Here is the transparency used to expose the photosenstive printed circuit board. It is actually laying on top of the board which as already been etched.
The resist is a blue color when it’s “active” and turns green after it has cured, and is no longer photosensitive.
Protecting eyes, ears and lungs is a very important step.
My drilling station is a basic dremel mototool (single speed) mounted in a drill mini-press. I have two 20 watt halogen lamps to illuminate the panel, one is a flood/fill light, the other a tight focus spot right on the cutting head. Using expensive aluminum titanium oxide coated solid carbide drill bits (designed for drilling fiberglass and non ferrous metals like titanium), the dremel easily pierces the board, spinning at 35000 RPM.
Lots of holes!
Make sure your safety gear is still on!
A visit to the impromptu depanelizing saw aka a Skill Jigsaw turned upside down and secured in a bench vice. With the blade installed backward, the foot of the saw provides a nice table for resting the panel on. This arrangment will gladly take a finger or at least mess you up, so make sure you have no distractions and always know where your fingers are while the blade is moving. Sometimes you’ll end up with pieces that are pretty small, but it’s not worth the risk of serious injury trying to do them on the saw… scoring both sides of the pcb with a drywall knife should let them snap cleanly… also I’ve heard large sheer-type paper cutters work.
The end result, a pile of little PCBs.
Sometimes the saw doesn’t leave the cleanest edge or your line is a bit wavey – a visit to mr belt-sander will clean things up nicely.
editors note: some of the pictures for this article didn’t come out as well as I had hoped, or have yet to be taken, so the [image missing] tag is a place holder to remind me to reshoot.
I’m already up past my bedtime, so just a few pictures for now. Write-up coming soon!
Specs: Input 2v to 6v DC … Output constant current 50mA up to 28v DC
Efficiency: Initial measurements, somewhere around 75%
Chip Texas Instruments TPS61040
size comparison, american quarter dollar piece
circuit detail – design is one-sided PCB with two through-hole jumper wires and the diode, everything else is smt
twin 1uF tantalum capacitors … the output capacitor I had originally selected was limited to 16v, so this was the best I could come up with on a Sunday. Note the top of the coil is missing – these things are fragile!
I may be easily amused, but I’m not easily impressed.
Everything I have read about pcb prototyping using photolithography claims I needed to use a laser printer and some sort of transparent or translucent medium. Armed with this knowledge, I used acetate, or overhead projector transparencies. The film claims to be designed for laser printers and copiers, but it still distorts some when printed on. Anyway, the problem with my laser printer is toner pin-holes. For whatever reason, the output would have these little holes everywhere, and these little holes would swiss-cheese my traces and ground plane pours. This forced me to use thick traces, which would still get swiss cheesed, but generally retained enough composure to be electrically conductive. The pin hole problem seems to be getting worse … the last test I performed on my printer as an exposure test of varying width lines and different sized holes and pay layouts. In general, the performance was pretty bad and was not acceptable for more advanced designed I wanted to make, involving very small SMT components.
So, I tried printing the same exposure test, on paper, to my inkjet printer. The results were stunning. The lines were sharper, the holes clearer, the pads better defined. What a difference it makes going from a 600 dpi laser to a 4800(?) dpi inkjet. So, I felt it worth the risk, and decided to run my exposure test with the inkjet. Using some inkjet transparencies (they’re kinda coated with some type of fiber?), I printed my patterns. I exposed my board, and then developed it… the results were WOW! Of course, I messed up a few things with this first run, mostly I let the light cook for too long. So in the best un-scientific manner possible, I changed a bunch of variables at the same time, and tried another pass.
That is the result! … Please ignore the greasy thumbprint on the left side of the board – that was acquired after etching, and has no effect on anything aside from mild embarrassment on my part. My setup was fairly simple. I printed my design at best quality, monochrome mode onto a transparency. Next, in my darkroom that doubles as a furnace room, I laid down my PCB, emulsion up, then the transparency, then a sheet of 1/4″ plate glass. About 4″ above that, I have two 15 watt under cabinet lights, each loaded with a GE Daylight bulb. The lights are connected to an extension cord, so I can turn them on and off together. After making sure everything is lined up, I start my stopwatch and plug in the lights. After letting it cook for eight minutes, I turned off the lights and slid the pcb into a waiting bath of developer. The developer had been sitting for about a day, so it was a little slow. I left the board sit for about 2 min, before turning on the room lights. As the emulsion started to dissolve, I could see the results were good, real good! With the room lights on, I stirred the developer and lightly brushed the pcb using a foam brush (as recommended by the manuf.) My image grew sharper and sharper.
After I was sure all the emulsion that needed to be gone was gone, I rinsed the board and slid it into a waiting bath of ferric chloride etchant. A few min later, I pulled the pcb out, to make sure all the copper had turned pink. If the copper is not pink, it means some emulsion remains, and its time for another trip to the developer. Fortunately all the copper was pink, no problems here. I let the pcb soak for about 30 min while I had some lunch. After lunch, and without washing my hands i might add, I extracted the PCB from the etchant and rinsed it off. The results are excellent.
So, no longer will I shy away from tiny SMTs, since I can now lay down traces as thin as 8 mils without issue (determined by the earlier exposure test). Granted there were three flaws I had to fix on this board, I suspect they were caused by either dust on the transparency or those fibers that are embedded in the plastic. A quick touch-up with the sharpie solved them without a hitch.
Next stop, de-panelize with the PCB “suicide” saw.
